Photolysis Techniques Research Articles

Copper(II) Complexes Derived from Schiff Bases Containing 4-Methylbenzylamine as a Core Unit: Cytotoxicity, pBR322-DNA Studies, Biological Assays, and Quantum Chemical ParametersCopper(II) Complexes Derived from Schiff Bases Containing 4-Methylbenzylamine as a Core Unit: Cytotoxicity, pBR322-DNA Studies, Biological Assays, and Quantum Chemical Parameters.

Bivalent copper complexes, [Cu(SB1)2] 1 (SB1 = (2-(4-methylbenzylimino)methyl)-5-methylphenol, [Cu(SB2)2] 2 (SB2 = (2-(4-methylbenzylimino)methyl)-4-bromolphenol), and [Cu(SB3)2] 3 (SB3 = (2-(4-methylbenzylimino)methyl)-4,6-dibromophenol) were synthesized using the Schiff bases prepared from 4-methylbenzylamine (p-tolylmethanamine). These were characterized using a variety of spectro-analytical methods. For all copper complexes, a square planar geometry was determined through spectral analyses. Utilizing molecular orbital energies, the stability of the copper complexes was calculated from quantum chemical characteristics. The kinetic and thermal degradation parameters were calculated from the thermograms. Studies on DNA binding interactions, such as UV absorption and emission, have shown that the manner of DNA binding is intercalative, and the binding constant (Kb) order is 3 > 2 > 1. Under oxidative and photolytic techniques, the copper complexes outperform the parent Schiff bases in their ability to cleave double-stranded pBR322 DNA. When tested for cytotoxicity on the KB3 and MCF7 cell lines, complexes displayed greater activity than their parent ligands. Studies on the complexes' in-vitro antibacterial and antioxidant activity showed that they are significantly more powerful than the parent ligands.

A Treatise on Furan Cored Schiff Base Cu(II), Ni(II) and Co(III) Complexes Accentuating Their Biological Efficacy: Synthesis, Thermal and Spectroscopic Characterization, DNA Interactions, Antioxidant and Antibacterial Activity Studies.

Three metal complexes [Cu(FMIMDIP)2 ] (1), [Ni(FMIMDIP)2 ] (2) and [Co(FMIMDIP)3 ] (3) where, FMIMDIP=(((furan-2-yl)methylimino)methyl)-4,6-diiodophenol, were synthesized and characterized by various spectroscopy. The analytical data revealed a square planar geometry for 1 and 2 and an octahedral geometry for 3. The kinetic and thermodynamic parameters of the thermal decomposition steps were calculated from the thermograms. The quantum chemical parameters have been calculated using HOMO-LUMO energies and reveal the stability of the complexes. The DNA interaction of 1-3 towards calf-thymus DNA was investigated by absorption titration, fluorescence spectroscopy and gel electrophoresis. All the complexes bind to DNA via intercalation mode with binding constant (Kb ) values of 4.17×103 M-1 to 5.9×104 M-1 and also effectively cleave pBR322 DNA by oxidative and photolytic techniques. The synergistic action of metal chelates with ascorbic acid induced the generation of free radicals. The antibacterial activity of 1-3 was tested against B. thuringiensis, S. pneumoniae, E. coli, and P. putida. Complex 3 has the best activity among all the complexes.

Bisphosphonic Acid‐Functionalized Water‐Soluble Photoinitiators

AbstractThe first bisphosphonic acid‐functionalized benzophenone (BP) and thioxanthone (TX) photoinitiators (PIs), BPBP and TXBP, are synthesized as a new class of water‐soluble PIs for free radical polymerization. BPBP shows excellent solubility (28 g L−1) in water at ambient temperature, compared to TXBP (0.05 g L−1) and also the commercial PI Irgacure 2959 (5 g L−1). BPBP and TXBP show UV–vis absorption at ≈260 (ε = 12912) and 405 (ε = 3314) nm in water. Photopolymerization results demonstrate that they can successfully initiate the photopolymerization of poly(ethylene glycol) diacrylate (Mn = 250 D) in the presence of bis‐(4‐tert‐butylphenyl)‐iodonium hexafluorophosphate (Iod). The photochemical mechanisms are investigated by electron‐spin resonance‐spin trapping, fluorescence, and steady‐state photolysis techniques.

Characterization of the nucleophilic reactivities of thiocarboxylate, dithiocarbonate and dithiocarbamate anions

The kinetics of the reactions of thiocarboxylate and thiocarbonate anions with benzhydrylium ions have been determined in acetonitrile solution using laser-flash photolytic techniques. The second-order rate constants (k) correlate linearly with the electrophilicity parameters E of the benzhydrylium ions, as required by the correlation log k (20 °C) = s(N)(N + E) (J. Am. Chem. Soc., 2001, 123, 9500-9512), allowing us to calculate the nucleophile-specific parameters N and s(N) for these anions. With these parameters, a direct comparison of the reactivities of thiocarboxylate, dithiocarbonate and dithiocarbamate anions with other nucleophiles becomes possible.

Matrix isolation and computational studies of the CF 2I radical

We report experimental and computational studies of the spectroscopy and photochemistry of the CF 2I radical, which was generated using pulsed discharge or photolytic techniques and trapped in Ar or Ne matrices held at ∼5 K. Ground state calculations were performed using UM062X and UB3LYP density functionals and the UMP2, UCCSD, and CASPT2 ab initio methods, all with high-quality basis sets. The photochemical reaction path from the lowest-lying electronically excited state was mapped using the CASPT2//CASSCF methodology. A conical intersection was found to drive the excited state species to the photoproduct/reactant wells on the ground state potential energy surface.

Electron Donor-Acceptor Interactions in Regioselectively Synthesized exTTF2-C70(CF3)10Dyads

The decakis(trifluoromethyl)fullerene C(1)-C(70)(CF(3))(10), in which the CF(3) groups are arranged on a para(7)-meta-para ribbon of C(6)(CF(3))(2) edge-sharing hexagons, and which has now been prepared in quantities of hundreds of milligrams, was reacted under standard Bingel-Hirsch conditions with a bis-pi-extended tetrathiafulvalene (exTTF) malonate derivative to afford a single exTTF(2)-C(70)(CF(3))(10) regioisomer in 80 % yield based on consumed starting material. The highly soluble hybrid was thoroughly characterized by using 1D (1)H, (13)C, and (19)F NMR, 2D NMR, and UV/Vis spectroscopy; matrix-assisted laser desorption ionization (MALDI) mass spectrometry; and electrochemistry. The cyclic voltammogram of the exTTF(2)-C(70)(CF(3))(10) dyad revealed an irreversible second reduction process, which is indicative of a typical retro-Bingel reaction; whereas the usual phenomenon of exTTF inverted potentials (E1ox>E2ox), resulting in a single, two-electron oxidation process, was also observed. Steady-state and time-resolved photolytic techniques demonstrated that the C(1)-C(70)(CF(3))(10) singlet excited state is subject to a rapid electron-transfer quenching. The resulting charge-separated states were identified by transient absorption spectroscopy, and radical pair lifetimes of the order of 300 ps in toluene were determined. The exTTF(2)-C(70)(CF(3))(10) dyad represents the first example of exploitation of the highly soluble trifluoromethylated fullerenes for the construction of systems able to mimic the photosynthetic process, and is therefore of interest in the search for new materials for photovoltaic applications.

Effect of hydrogen donating ability of benzofuranone on the antioxidant activity

The rate constants for the hydrogen donation of three 3-aryl-benzofuranone compounds with tert-butoxyl radicals have been calculated through laser flash photolysis (LFP) technique, which were used to evaluate the hydrogen donating abilities of those 3-aryl-benzofuranone compounds. The antioxidant activities of those 3-aryl-benzofuranone compounds in polypropylene (PP) have been studied using the oxidation induction time (OIT) of PP stabilized by those 3-aryl-benzofuranone compounds too. It has been found that the methyl group on the 2′-position can prevent the hydrogen donation of 3-aryl-benzofuranone with tert-butoxyl radicals and weakens the antioxidant activity of 3-aryl-benzofuranone in the oxidation of PP, i.e., the greater is the hydrogen donating ability of 3-aryl-benzofuranone with radicals, the better is the antioxidant activity of 3-aryl-benzofuranone in the oxidation of PP in the presence of oxygen.

Technical note: Measuring tropospheric OH and HO<sub>2</sub> by laser-induced fluorescence at low pressure. A comparison of calibration techniques

Abstract. The hydroxyl radical (OH) is one of the most important oxidants in the atmosphere, as it is involved in many reactions that affect regional air quality and global climate change. Because of its high reactivity, measurements of OH radical concentrations in the atmosphere are difficult, and often require careful calibrations that rely on the production of a known concentration of OH at atmospheric pressure. The Indiana University OH instrument, based on the Fluorescence Assay by Gas Expansion technique (FAGE), has been calibrated in the laboratory using two different approaches: the production of OH from the UV-photolysis of water-vapor, and the steady-state production of OH from the reaction of ozone with alkenes. The former technique relies on two different actinometric methods to measure the product of the lamp flux at 184.9 nm and the photolysis time. This quantity derived from N2O actinometry was found to be 1.5 times higher than that derived from O2 actinometry. The water photolysis and ozone-alkene techniques are shown to agree within their experimental uncertainties (respectively 17% and 44%), although the sensitivities derived from the ozone-alkene technique were systematically lower by 40% than those derived from the water-vapor UV- photolysis technique using O2 actinometry. The agreement between the two different methods improves the confidence of the water-vapor photolysis method as an accurate calibration technique for HOx instruments. Because several aspects of the mechanism of the gas phase ozonolysis of alkenes are still uncertain, this technique should be used with caution to calibrate OH instruments.

Energy Transfer in Oligofluorene-C60 and C60-Oligofluorene-C60 Donor−Acceptor Conjugates

Two complementary series of C(60)-(Fl)(n) and C(60)-(Fl)(n)-C60 (Fl = 9,9-dihexylfluorene-2,7-diyl; n = 1-5) derivatives with terminal N-methylfulleropyrrolidine units have been synthesized from CHO-(Fl)(n) and CHO-(Fl)(n)-CHO precursors via 1,3-dipolar cycloaddition reaction of in situ generated azomethine ylides with an excess of C60. In solution electrochemical experiments, these conjugates give rise to amphoteric redox behavior. Three consecutive quasireversible reduction waves have been observed at the expected potentials for the N-methylfulleropyrrolidine cores. For the C(60)-(Fl)(n)-C(60) series, each reduction wave is a two-electron process with no observable interaction between the C(60) units. Two or, in some cases, three oxidation waves--most of them irreversible--are ascribed to the oligofluorene system. These waves are cathodically shifted with an increasing number of fluorene units and anodically shifted by the conjugated terminal aldehyde units, compared to the N-methylfulleropyrrolidine termini. Steady-state and time-resolved photolytic techniques show that an efficient transduction of singlet excited-state energy transfer prevails from the photoexcited oligofluorene to the energy accepting fullerene.

CH radical production from 248nm photolysis or discharge-jet dissociation of CHBr3 probed by cavity ring-down absorption spectroscopy

The A-X bands of the CH radical, produced in a 248 nm two-photon photolysis or in a supersonic jet discharge of CHBr(3), have been observed via cavity ring-down absorption spectroscopy. Bromoform is a well-known photolytic source of CH radicals, though no quantitative measurement of the CH production efficiency has yet been reported. The aim of the present work is to quantify the CH production from both photolysis and discharge of CHBr(3). In the case of photolysis, the range of pressure and laser fluences was carefully chosen to avoid postphotolysis reactions with the highly reactive CH radical. The CH production efficiency at 248 nm has been measured to be Phi=N(CH)N(CHBr(3))=(5.0+/-2.5)10(-4) for a photolysis laser fluence of 44 mJ cm(-2) per pulse corresponding to a two-photon process only. In addition, the internal energy distribution of CH(X (2)Pi) has been obtained, and thermalized population distributions have been simulated, leading to an average vibrational temperature T(vib)=1800+/-50 K and a rotational temperature T(rot)=300+/-20 K. An alternative technique for producing the CH radical has been tested using discharge-induced dissociation of CHBr(3) in a supersonic expansion. The CH product was analyzed using the same cavity ring-down spectroscopy setup. The production of CH by discharge appears to be as efficient as the photolysis technique and leads to rotationally relaxed radicals.

Photoinduced Carbon Monoxide Migration in a Synthetic Heme−Copper Complex

Time-resolved infrared (TRIR) flash photolytic techniques have been employed to initiate and observe the efficient dissociation of CO from a synthetic heme-CO/copper complex, [((6)L)Fe(II)(CO)..Cu(I)](+) (2), in CH(3)CN and acetone at room temperature. In CH(3)CN, a significant fraction of the photodissociated CO molecules transiently bind to copper (nu(CO)(Cu) = 2091 cm(-)(1)) giving [((6)L)Fe(II)..Cu(I)(CO)](+) (4), with an observed rate constant, k(1) = 1.5 x 10(5) s(-)(1). That is followed by a slower direct transfer of CO from the copper moiety back to the heme (nu(CO)(Fe) = 1975 cm(-)(1)) with k(2) = 1600 s(-)(1). Additional transient absorption (TA) UV-vis spectroscopic experiments have been performed monitoring the CO-transfer reaction by following the Soret band. Eyring analysis of the temperature-dependent data yields DeltaH(double dagger) = 43.9 kJ mol(-)(1) for the 4-to-2 transformation, similar to that for CO dissociation from [Cu(I)(tmpa)(CO)](+) in CH(3)CN (DeltaH(double dagger) = 43.6 kJ mol(-)(1)), suggesting CO dissociation from copper regulates the binding of small molecules to the heme within [((6)L)Fe(II)..Cu(I)](+)(3). Our observations are analagous to those observed for the heme(a3)/Cu(B) active site of cytochrome c oxidase, where photodissociated CO from the heme(a3) site immediately (ps) transfers to Cu(B) followed by millisecond transfer back to the heme.

Photophysical and photochemical study of laser-dye coumarin-481 and its photoproduct in solution

An ultraviolet light-induced photophysical and Photochemical changes of coumarin-481 in cyclohexane have been studied by photolysis technique at room temperature, due to its potential applications in photonics, photochemistry, and electronic spectroscopy. During the optical pumping, coumarin-481 showed photochemical changes, therefore as the concentration of coumarin-481 decreased, a photoproduct concentration increased rapidly. An absorption band of the product was observed at around 250 nm. Photoproduct emission spectra characteristics show that photoproduct molecules can also be used as a laser-dye at different emission frequency.

Substituent and solvent effects on the N‐2—N‐3 hindered rotation of cis‐1,3‐diphenyltriazenes

AbstractLaser‐flash photolysis techniques were applied to investigate the dependence of the rotational barrier about the N‐2—N‐3 bond of symmetrically 4,4′‐disubstituted cis‐1,3‐diphenyltriazenes on substituents and solvents. The increase in the rotational barrier with increasing ability of the 4‐substituent to withdraw electrons implies the intramolecular process to be more susceptible to the electronic character of the aryl group attached to N‐1 than of that bonded to N‐3. Furthermore, the increase in the rotational barrier with decreasing solvent polarity implies an increase in dipole moment on rotation from the ground state to the transition state. Copyright © 2004 John Wiley & Sons, Ltd.

Probing charge separation in structurally different C60/exTTF ensembles.

The scope of the present work is to highlight the effects stemming from different C60/exTTF linkages (exTTF = 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene)-either via an anthracene unit or a dithiole ring. Particular emphasis is placed on photoinduced electron-transfer features. Therefore, we devised a new series of C60-exTTF ensembles, synthesized via 1,3-dipolar cycloaddition and Diels-Alder cycloaddition reactions, in which exTTF units are separated from C60 by two single bonds (3a-c, 4), one vinylene unit (5a), or two vinylene units (5b). The cyclic voltammetry reveals an amphoteric redox behavior with remarkably strong electron-donor ability of the trimethyl-substituted exTTF moiety in 4 and 5a,b. Steady-state and time-resolved photolytic techniques show that the fullerene singlet excited state in (3a-c, 4, and 5a,b) is subject to a rapid electron-transfer quenching. The resulting charge-separated states, that is C60*(-)-exTTF*+, were identified by transient absorption spectroscopy. We determined radical pair lifetimes of the order of 200 ns in benzonitrile. This suggests (i) that the positive charge of the exTTF*+ is delocalized over the entire donor rather than localized on one of the 1,3-dithiole rings and (ii) that linking exTTF via the anthracene or 1,3-dithiole ring has no appreciable influence. Increasing the donor-acceptor separation via implementing one or two vinylene units as spacers led to improved radical pair lifetimes (5a: tau = 725 ns; 5b: tau = 1465 ns).

Sulfur ylides generated from the reaction of adamantylidene and phenylcarbene with sulfur substrates

Reaction of adamantylidene and phenylcarbene with ethylthiol, ethylene dithiol, allylethylsulfide, allylphenylsulfide, and trimethylenesulfide involves the formation of a sulfur ylide intermediate, followed by H-migration, 2,3-sigmatropic shift, or ring opening to give sulfides. The sulfur ylide formed in the reaction of phenylcarbene with trimethylenesulfide is directly observed by laser flash photolytic techniques.

Controlling short- and long-range electron transfer processes in molecular dyads and triads.

Novel pi-extended tetrathiafulvalene (exTTF)-based donor acceptor hybrids-dyads and triads-have been synthesized following a multistep synthetic procedure. Cyclic voltammetry and absorption spectroscopy, conducted in room temperature solutions, reveal features that are identical to the sum of the separate donor and acceptor moieties. Steady-state and time-resolved photolytic techniques confirm that upon photoexcitation of the fullerene chromophore, rapid (1.25 x 10(10) s(-1)) and efficient (67 %) charge separation leads to long-lived, charge-separated radical pairs. Typical lifetimes for the dyad ensembles range between 54 and 460 ns, with the longer values found in more polar solvents. This indicates that the dynamics are located in the 'normal region' of the Marcus curve. In the triads, subsequent charge shifts transform the adjacent radical pair into the distant radical pair, for which we determined lifetimes of up to 111 micros in DMF-values never previously accomplished in molecular triads. In the final charge-separated state, large donor-acceptor separation (center-to-center distances: approximately 30 A) minimizes the coupling between reduced acceptor and oxidized donor. Analysis of the charge recombination kinetics shows that a stepwise mechanism accounts for the unusually long lifetimes.

Antioxidative Properties of Martynoside: Pulse Radiolysis and Laser Photolysis Study

Free radical reactions of Martynoside (MAR), a phenylpropanoid glycoside, with a variety of oxidants were studied in the aqueous solution by laser photolysis and pulse radiolysis techniques. The pKa value of MAR in aqueous solution was measured from the pH dependent changes of the UV absorption at 384 nm with value of pKa=9.2. The phenoxyl radical of MAR which exhibits maximum absorption at 360 nm was generated by one-electron transfer to N3• or Br2•-. Other important properties of phenoxyl radical such as extinction coefficient, formation and decay rate constants were also determined. The reaction rate constant of O2•- with MAR, k=8.5×104 dm3 · mol-1 · s-1, was measured by the method of competition kinetics. By measuring time-resolved luminescence emission at 1270 nm, the quenching rate constant of singlet oxygen by MAR was obtained to be 3.3×106 dm3 · mol-1 · s-1. Reduction potential of the MAR couple (MAR•/MAR), determined using rutin as reference compound, gave a value E=0.66 V vs. NHE. The antioxidative properties of MAR were compared with those of some well-known antioxidants.

Generation and identification of the transient intermediates of allophycocyanin by laser photolytic and pulse radiolytic techniques

Purpose : To explore the photophysical and photochemical properties of allophycocyanin (APC) and contribute to the understanding of the molecular mechanism of APC photosensitization. Materials and methods : Laser-flash photolytic and pulse radiolytic techniques were used for the first time to characterize the transient intermediates involved in APC photochemistry and photophysics. The excited triplet state and radical cation of APC were identified by acetone sensitization and one-electron oxidation. Results : The 248-nm laser-flash photolysis of APC in N 2 -saturated aqueous solution (pH 7.0) yields the triplet state and radical cation of APC. The APC radical cations were generated by ionization via a monophotonic process, with a quantum yield of 0.17. Conclusions : APC can undergo both photo-excitation and photo-ionization under the present experimental conditions. These new findings suggest that APC has the potential to act as both a type I and type II photosensitizer.

EPR investigation of the free radicals generated during the photosensitization of TiO2 colloid by hypocrellin B

The cation radical of dye produced from the interfacial electron transfer from a surface chelated dye to the conduction band of the colloidal TiO2 was studied by laser flash photolysis and electron paramagnetic resonance (EPR) techniques. The study employed hypocrellin B (HB), a natural photodynamic pigment with strong absorption over the visible light region, as a sensitizer and titanium dioxide as a colloid semiconductor. HB formed a chelate with this colloid semiconductor and exhibited a red-shifted and strongly enhanced absorption in the visible spectrum. Laser photolysis indicated that the electron excitation in the visible absorption band of the chelate resulted in extremely rapid and efficient electron injection from the excited triplet state of the dye into the conduction band of the semiconductor. A transient absorption of cation radical of HB at 570 nm was observed. The appearance of cation radical of HB was characterized by EPR spectrometry: the photoinduced EPR signal was not quenched by oxygen and its intensity decreased in the presence of NaI, a typical hole scavenger. The generation of conduction band electrons in HB-sensitized TiO2 system was also verified by the spin elimination of a stable cyclic nitroxide, 2,2, 6,6-tetramethylpiperidine-1-oxyl (TEMPO), and by the reduction of methyl viologen (MV2+) to its radical MV+·.

Sulfur ylides generated from the reaction of arylchlorocarbenes with trimethylenesulfide: determination of rate parameters by laser flash photolysis

Reaction of arylchlorodiazirines with trimethylenesulfide gives a mixture of aryldi(3-chloropropyl)thioacetal and aryl(2-propenyl)(3-chloropropyl)thioacetal in a good yield. The reaction goes through a formation of a sulfur ylide as an intermediate. Rate constants have been determined using laser flash photolytic techniques.